Lithographic apparatus and device manufacturing method

ABSTRACT

A method and apparatus for monitoring a level of silicon dioxide in a liquid and removing the silicon dioxide using polishers is disclosed. In an embodiment, two polishers that absorb carbon dioxide and silicon dioxide, but which have a greater affinity for carbon dioxide, are placed in series along a conduit containing the liquid for use in an immersion lithographic apparatus. The upstream polisher absorbs carbon dioxide and silicon dioxide until it is saturated, at which point it desorbs the silicon dioxide in preference for the carbon dioxide. Silicon dioxide continues down the conduit and is absorbed by the downstream polisher. Once the upstream polisher is saturated with carbon dioxide, carbon dioxide present in the liquid flows downstream where it is absorbed by the downstream polisher. A sensor between the polishers senses the presence of carbon dioxide and initiates a request for the one or more of polishers to be cleaned or replaced.

FIELD

The present invention relates to a lithographic apparatus and a methodfor manufacturing a device. In particular, the present invention relatesto an immersion system, in which a liquid is confined between aprojection system and a substrate to be exposed by the projectionsystem.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.comprising part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

It has been proposed to immerse the substrate in the lithographicprojection apparatus in a liquid having a relatively high refractiveindex, e.g. water, so as to fill a space between the final element ofthe projection system and the substrate. The point of this is to enableimaging of smaller features since the exposure radiation will have ashorter wavelength in the liquid. (The effect of the liquid may also beregarded as increasing the effective NA of the system and alsoincreasing the depth of focus.) Other immersion liquids have beenproposed, including water with solid particles (e.g. quartz) suspendedtherein.

However, submersing the substrate or substrate and substrate table in abath of liquid (see, for example, U.S. Pat. No. 4,509,852, herebyincorporated in its entirety by reference) means that there is a largebody of liquid that must be accelerated during a scanning exposure. Thisrequires additional or more powerful motors and turbulence in the liquidmay lead to undesirable and unpredictable effects.

One of the solutions proposed is for a liquid supply system to provideliquid on only a localized area of the substrate and in between thefinal element of the projection system and the substrate using a liquidconfinement system (the substrate generally has a larger surface areathan the final element of the projection system). One way which has beenproposed to arrange for this is disclosed in PCT patent application WO99/49504, hereby incorporated in its entirety by reference. Asillustrated in FIGS. 2 and 3, liquid is supplied by at least one inletIN onto the substrate, preferably along the direction of movement of thesubstrate relative to the final element, and is removed by at least oneoutlet OUT after having passed under the projection system. That is, asthe substrate is scanned beneath the element in a −X direction, liquidis supplied at the +X side of the element and taken up at the −X side.FIG. 2 shows the arrangement schematically in which liquid is suppliedvia inlet IN and is taken up on the other side of the element by outletOUT which is connected to a low pressure source. In the illustration ofFIG. 2 the liquid is supplied along the direction of movement of thesubstrate relative to the final element, though this does not need to bethe case. Various orientations and numbers of in- and out-letspositioned around the final element are possible, one example isillustrated in FIG. 3 in which four sets of an inlet with an outlet oneither side are provided in a regular pattern around the final element.

A further immersion lithography solution with a localized liquid supplysystem is shown in FIG. 4. Liquid is supplied by two groove inlets IN oneither side of the projection system PL and is removed by a plurality ofdiscrete outlets OUT arranged radially outwardly of the inlets IN. Theinlets IN and OUT can be arranged in a plate with a hole in its centerand through which the projection beam is projected. Liquid is suppliedby one groove inlet IN on one side of the projection system PL andremoved by a plurality of discrete outlets OUT on the other side of theprojection system PL, causing a flow of a thin film of liquid betweenthe projection system PL and the substrate W. The choice of whichcombination of inlet IN and outlets OUT to use can depend on thedirection of movement of the substrate W (the other combination of inletIN and outlets OUT being inactive).

SUMMARY

As shown in FIGS. 2, 3, 4 and 5 (described in more detail below), whilethe substrate is being exposed, it comes into contact with the liquidheld in the liquid confinement system. When the substrate is removed andis dried, particles in the liquid may be deposited on the surface of thesubstrate. These particles could cause defects in the pattern printed onthe circuit, potentially rendering it unusable, particularly sincemethods used to remove the particles may remove layers of the pattern onthe surface of the substrate also.

In order to purify the liquid to ensure that there are no bacteria, etc.in it, the liquid is treated with UV radiation and also undergoes “TotalOrganic Content” (TOC) removal. However, these do not remove or evensense the presence of silicon dioxide particles and furthermore, the UVradiation might be responsible for the presence of silicon dioxide bycausing silicon to be converted into silicon dioxide, the silicon beingpresent in the liquid because it erodes off the silicon-based substrate.The liquid therefore should be monitored for its silicon dioxide(silica) content. However, monitoring silica levels by taking samples ofthe liquid at regular intervals and analyzing these separately islabor-intensive and unreliable and requires logistical organization ofpeople and laboratories. Furthermore, there is a delay between thesample being taken and the result of the analysis found, during whichsilica in the liquid may have already had a detrimental effect on asubstrate.

One potential solution is to use in-line silica sensors, but these areexpensive and complex and not sensitive enough to detect the smallamounts of silica that could be detrimental to a substrate pattern. Therequired sensitivity of a silica sensor would be approximately 10 ppt.

Clearly, the silica should be removed. This may be done with liquidpolishers in the liquid flow path. In an embodiment, the liquidpolishers may be water polishers. Water polishers use ion exchangers toremove contaminants such as silica, etc. from ultra pure water (UPW).The polishers have a layer of ion-exchanging resin, which absorbssilica. This is a natural property of the resins that are commonly usedin water polishers and is a consequence of the chemical reactionsinvolved in the resins.

A problem with the polishers is that when the resin becomes saturated,it stops absorbing the problematic silica. This often happens suddenlyso that there is no warning of the sudden presence of potentially highlevels of silica in the liquid. There may then be no assurance ofmeasuring the liquid at the right time to test for silica levels beforesilica particles are potentially deposited on the substrate.

Accordingly, it would be desirable, for example, to prevent or reducethe deposition of silicon dioxide particles on a substrate surface bydetecting and removing silicon dioxide from the liquid in the immersionsystem.

According to an aspect of the present invention, there is provided anapparatus for removing silicon dioxide from liquid (and indirectlymonitoring the level of silicon dioxide in the liquid), comprising aconduit configured to carry the liquid; a first polisher; a sensor; anda second polisher. The first polisher and the second polisher arearranged in series along the conduit so that the first polisher isupstream of the sensor, which in turn is upstream of the secondpolisher. The first and second polishers are configured to absorb carbondioxide and silicon dioxide and have a greater affinity for carbondioxide than for silicon dioxide. The sensor is configured to sense thepresence of carbon dioxide in the liquid at a point between the twopolishers.

According to another aspect of the present invention, there is provideda lithographic apparatus comprising a projection system; a substratetable configured to hold a substrate to be exposed by a radiation beamfrom the projection system; a liquid supply system configured to supplyliquid to a liquid confinement system that is configured to confineliquid between the projection system and the substrate; and an apparatusconfigured to remove silicon dioxide from the liquid (and indirectlymonitoring the presence of silicon dioxide in the liquid) as describedabove.

According to another aspect of the present invention, there is provideda method of removing silicon dioxide from liquid, by sensing thepresence of carbon dioxide in a conduit between an upstream and adownstream polisher, the polishers absorbing silicon dioxide and carbondioxide from the liquid in the conduit.

According to yet another aspect of the present invention, there isprovided a method of removing silicon dioxide from the liquid,comprising providing a conduit for carrying the liquid; providing afirst and second polisher along the conduit, the second polisher beingdownstream from the first polisher; providing a sensor for sensingcarbon dioxide between the first and second polishers on the conduit;providing the liquid in the conduit; the first polisher absorbingsilicon dioxide and carbon dioxide until it is saturated; the firstpolisher absorbing carbon dioxide and desorbing the absorbed silicondioxide; the second polisher absorbing the silicon dioxide desorbed bythe first polisher; the first polisher becoming saturated with carbondioxide and ceasing to absorb carbon dioxide; the sensor sensing carbondioxide in the liquid and transmitting a signal indicating that thepolishers are saturated; and initiating a request that the polishers arereplaced or cleaned; and the second polisher absorbing silicon dioxideand carbon dioxide until it is saturated, replaced or cleaned.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIGS. 2 and 3 depict a liquid supply system for use in a lithographicprojection apparatus;

FIG. 4 depicts a another liquid supply system for use in a lithographicprojection apparatus;

FIG. 5 depicts an immersion head according to an embodiment of theinvention; and

FIGS. 6 a, 6 b and 6 c show the various steps according to an embodimentof a method of the invention.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus according to oneembodiment of the invention. The apparatus comprises:

-   -   an illumination system (illuminator) IL configured to condition        a radiation beam B (e.g. UV radiation or DUV radiation).    -   a support structure (e.g. a mask table) MT constructed to        support a patterning device (e.g. a mask) MA and connected to a        first positioner PM configured to accurately position the        patterning device in accordance with certain parameters;    -   a substrate table (e.g. a wafer table) WT constructed to hold a        substrate (e.g. a resist-coated wafer) W and connected to a        second positioner PW configured to accurately position the        substrate in accordance with certain parameters; and    -   a projection system (e.g. a refractive projection lens system)        PS configured to project a pattern imparted to the radiation        beam B by patterning device MA onto a target portion C (e.g.        comprising one or more dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure supports, i.e. bears the weight of, the patterningdevice. It holds the patterning device in a manner that depends on theorientation of the patterning device, the design of the lithographicapparatus, and other conditions, such as for example whether or not thepatterning device is held in a vacuum environment. The support structurecan use mechanical, vacuum, electrostatic or other clamping techniquesto hold the patterning device. The support structure may be a frame or atable, for example, which may be fixed or movable as required. Thesupport structure may ensure that the patterning device is at a desiredposition, for example with respect to the projection system. Any use ofthe terms “reticle” or “mask” herein may be considered synonymous withthe more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more support structures). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may comprise an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL maycomprise various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross-section.

The radiation beam B is incident on the patterning device (e.g., mask)MA, which is held on the support structure (e.g., mask table) MT, and ispatterned by the patterning device. Having traversed the patterningdevice MA, the radiation beam B passes through the projection system PS,which focuses the beam onto a target portion C of the substrate W. Withthe aid of the second positioner PW and position sensor IF (e.g. aninterferometric device, linear encoder or capacitive sensor), thesubstrate table WT can be moved accurately, e.g. so as to positiondifferent target portions C in the path of the radiation beam B.Similarly, the first positioner PM and another position sensor (which isnot explicitly depicted in FIG. 1) can be used to accurately positionthe patterning device MA with respect to the path of the radiation beamB, e.g. after mechanical retrieval from a mask library, or during ascan. In general, movement of the support structure MT may be realizedwith the aid of a long-stroke module (coarse positioning) and ashort-stroke module (fine positioning), which form part of the firstpositioner PM. Similarly, movement of the substrate table WT may berealized using a long-stroke module and a short-stroke module, whichform part of the second positioner PW. In the case of a stepper (asopposed to a scanner) the support structure MT may be connected to ashort-stroke actuator only, or may be fixed. Patterning device MA andsubstrate W may be aligned using patterning device alignment marks M1,M2 and substrate alignment marks P1, P2. Although the substratealignment marks as illustrated occupy dedicated target portions, theymay be located in spaces between target portions (these are known asscribe-lane alignment marks). Similarly, in situations in which morethan one die is provided on the patterning device MA, the patterningdevice alignment marks may be located between the dies.

The depicted apparatus could be used in at least one of the followingmodes:

-   -   1. In step mode, the support structure MT and the substrate        table WT are kept essentially stationary, while an entire        pattern imparted to the radiation beam is projected onto a        target portion C at one time (i.e. a single static exposure).        The substrate table WT is then shifted in the X and/or Y        direction so that a different target portion C can be exposed.        In step mode, the maximum size of the exposure field limits the        size of the target portion C imaged in a single static exposure.    -   2. In scan mode, the support structure MT and the substrate        table WT are scanned synchronously while a pattern imparted to        the radiation beam is projected onto a target portion C (i.e. a        single dynamic exposure). The velocity and direction of the        substrate table WT relative to the support structure MT may be        determined by the (de-)magnification and image reversal        characteristics of the projection system PS. In scan mode, the        maximum size of the exposure field limits the width (in the        non-scanning direction) of the target portion in a single        dynamic exposure, whereas the length of the scanning motion        determines the height (in the scanning direction) of the target        portion.    -   3. In another mode, the support structure MT is kept essentially        stationary holding a programmable patterning device, and the        substrate table WT is moved or scanned while a pattern imparted        to the radiation beam is projected onto a target portion C. In        this mode, generally a pulsed radiation source is employed and        the programmable patterning device is updated as required after        each movement of the substrate table WT or in between successive        radiation pulses during a scan. This mode of operation can be        readily applied to maskless lithography that utilizes        programmable patterning device, such as a programmable mirror        array of a type as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

Another solution which has been proposed is to provide the liquid supplysystem with a liquid confinement structure which extends along at leasta part of a boundary of the space between the final element of theprojection system and the substrate table. Such a solution isillustrated in FIG. 5. The liquid confinement structure is substantiallystationary relative to the projection system in the XY plane thoughthere may be some relative movement in the Z direction (in the directionof the optical axis). A seal is formed between the liquid confinementstructure and the surface of the substrate. Preferably the seal is acontactless seal such as a gas seal. Such as system with a gas seal isdisclosed in United States patent application publication no. US2004-0207824, hereby incorporated in its entirety by reference.

FIG. 5 shows the immersion head IH of FIG. 1 in more detail. Itincorporates a contactless seal to keep liquid 11 within the immersionhead reservoir 10. The reservoir 10 forms a contactless seal to thesubstrate around the image field of the projection system so that liquidis confined to fill a space between the substrate surface and the finalelement of the projection system. The reservoir is formed by a liquidconfinement structure 12 positioned below and surrounding the finalelement of the projection system PL. Liquid is brought into the spacebelow the projection system and within the liquid confinement structure12. The liquid confinement structure 12 extends a little above the finalelement of the projection system and the liquid level rises above thefinal element so that a buffer of liquid is provided. The liquidconfinement structure 12 has an inner periphery that at the upper endpreferably closely conforms to the shape of the projection system or thefinal element thereof and may, e.g., be round. At the bottom, the innerperiphery closely conforms to the shape of the image field, e.g.,rectangular though this need not be the case.

The liquid is confined in the reservoir by a gas seal 16 between thebottom of the liquid confinement structure 12 and the surface of thesubstrate W. The gas seal is formed by gas, e.g. air or synthetic airbut preferably N₂ or another inert gas, provided under pressure viainlet 15 to the gap between liquid confinement structure 12 andsubstrate and extracted via first outlet 14. The overpressure on the gasinlet 15, vacuum level on the first outlet 14 and geometry of the gapare arranged so that there is a high-velocity air flow inwards thatconfines the liquid.

FIGS. 6 a, 6 b and 6 c show the various steps involved in detecting andremoving silicon dioxide from the liquid as it travels to the immersionhead reservoir 10. A TOC remover/UV lamp 40 is located upstream of theimmersion head 10. Downstream of the UV lamp 40 is a first polisher 20,then a conductivity sensor 30 and finally a second polisher 22. As theTOC remover/UV lamp 40 is deployed, a by-product of TOC removal iscarbon dioxide. The polishers 20 and 22 comprise an ion-exchange resinthat absorbs carbon dioxide as well as silicon dioxide. As shown in FIG.6 a, at the beginning of the process, polisher 20 absorbs both carbondioxide and silicon dioxide.

Polisher 20 will absorb carbon dioxide and silicon dioxide until it issaturated. At this point, because the resin of the polisher 20 has agreater affinity for carbon dioxide than for silicon dioxide, thepolisher will continue to absorb carbon dioxide and begin to desorbsilicon dioxide. Silicon dioxide therefore is returned into the liquidand is subsequently absorbed by polisher 22. This absorption of carbondioxide and desorption of silicon dioxide goes on until polisher 20 issaturated with carbon dioxide. This is shown in FIG. 6 c. Polisher 20 isfully saturated and no longer absorbs either carbon dioxide or silicondioxide. Carbon dioxide and silicon dioxide therefore continue down theconduit to polisher 22 which is still absorbing both carbon dioxide andsilicon dioxide. Because these compounds are being absorbed by thesecond polisher 22, the liquid reaching the immersion head 10 is stillfree from undesirable particles. Because there is now carbon dioxide inthe liquid downstream of the first polisher, the conductivity sensor 30senses the carbon dioxide in the liquid and sends a signal indicatingthat one or more of the polishers are saturated and a request for theone or more of the polishers to be replaced or cleaned.

The size of the second polisher 22 can be balanced with the size of thefirst polisher 20 such that there is time to change one or morepolishers once sensor 30 has detected carbon dioxide before polisher 22becomes saturated as well.

In other words, there is indirect measurement of silicon dioxide bymeasuring carbon dioxide levels. Carbon dioxide is far simpler to detectthan silicon dioxide and can be done on-line. This method thereforeprovides robust on-line monitoring of silicon dioxide in liquid supplysystem liquid using industry standard equipment.

In European Patent Application No. 03257072.3 the idea of a twin or dualstage immersion lithography apparatus is disclosed. Such an apparatus isprovided with two tables for supporting one or more substrates. Levelingmeasurements are carried out with a table at a first position, withoutimmersion liquid, and exposure is carried out with a table at a secondposition, where immersion liquid is present. Alternatively, theapparatus has only one table.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 248, 193, 157 or 126 nm).

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, including refractiveand reflective optical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having sucha computer program stored therein.

One or more embodiments of the invention may be applied to any immersionlithography apparatus, in particular, but not exclusively, those typesmentioned above and whether the immersion liquid is provided in the formof a bath or only on a localized surface area of the substrate. A liquidsupply system as contemplated herein should be broadly construed. Incertain embodiments, it may be a mechanism or combination of structuresthat provides a liquid to a space between the projection system and thesubstrate and/or substrate table. It may comprise a combination of oneor more structures, one or more liquid inlets, one or more gas inlets,one or more gas outlets, and/or one or more liquid outlets that provideliquid to the space. In an embodiment, a surface of the space may be aportion of the substrate and/or substrate table, or a surface of thespace may completely cover a surface of the substrate and/or substratetable, or the space may envelop the substrate and/or substrate table.The liquid supply system may optionally further include one or moreelements to control the position, quantity, quality, shape, flow rate orany other features of the liquid.

The immersion liquid used in the apparatus may have differentcompositions, according to the desired properties and the wavelength ofexposure radiation used. For an exposure wavelength of 193 nm, ultrapure water or water-based compositions may be used and for this reasonthe immersion liquid is sometimes referred to as water and water-relatedterms such as hydrophilic, hydrophobic, humidity, etc. may be used.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

1. An apparatus for removing silicon dioxide from a liquid, comprising:a conduit configured to carry the liquid; a first polisher; a sensor;and a second polisher, wherein the first polisher and the secondpolisher are arranged in series along the conduit so that the firstpolisher is upstream of the sensor which is upstream of the secondpolisher, the first and second polishers are configured to absorb carbondioxide and silicon dioxide and have a greater affinity for carbondioxide than for silicon dioxide, and the sensor is configured to sensethe presence of carbon dioxide in the liquid at a point between thefirst and second polishers.
 2. The apparatus of claim 1, wherein thefirst and second polishers comprise an ion-exchanging resin with agreater affinity for carbon dioxide than silicon dioxide.
 3. Theapparatus of claim 1, wherein the sensor is a conductivity sensor. 4.The apparatus of claim 1, further comprising a UV lamp upstream of thefirst polisher.
 5. The apparatus of claim 1, further comprising a TOCremover upstream of the first polisher.
 6. The apparatus of claim 1,wherein the liquid is ultra pure water.
 7. A lithographic apparatus,comprising: a projection system; a substrate table configured to hold asubstrate to be exposed by a radiation beam from the projection system;a liquid supply system configured to supply liquid to a liquidconfinement system, the liquid confinement system configured to confineliquid between the projection system and the substrate; and an apparatusconfigured to remove silicon dioxide from the liquid, comprising: aconduit for carrying the liquid; a first polisher; a sensor; and asecond polisher, wherein the first polisher and the second polisherarranged in series along the conduit so that the first polisher isupstream of the sensor which is upstream of the second polisher, thefirst and second polishers are configured to absorb carbon dioxide andsilicon dioxide and have a greater affinity for carbon dioxide than forsilicon dioxide, and the sensor is configured to sense the presence ofcarbon dioxide in the liquid at a point between the two polishers. 8.The apparatus of claim 7, wherein the first and second polisherscomprise an ion-exchanging resin with a greater affinity for carbondioxide than silicon dioxide.
 9. The apparatus of claim 7, wherein thesensor is a conductivity sensor.
 10. The apparatus of claim 7, furthercomprising a UV lamp upstream of the first polisher.
 11. The apparatusof claim 7, further comprising a TOC remover upstream of the firstpolisher.
 12. The apparatus of claim 7, wherein the liquid is ultra purewater.
 13. A method of removing silicon dioxide from liquid, comprisingsensing the presence of carbon dioxide in a conduit between an upstreamand a downstream polisher, the polishers absorbing silicon dioxide andcarbon dioxide from liquid in the conduit.
 14. The method of claim 13,wherein the upstream polisher has a greater affinity for carbon dioxidethan silicon dioxide such that once the polisher is saturated withcarbon dioxide and silicon dioxide, it desorbs silicon dioxide in orderto absorb more carbon dioxide and once it is saturated with carbondioxide, it stops absorbing either silicon dioxide or carbon dioxide,and the sensor senses carbon dioxide in the liquid.
 15. The method ofclaim 13, wherein the sensor initiates a request that one or more of thepolishers are replaced or cleaned, and a downstream polisher absorbssilicon dioxide and carbon dioxide until it is saturated, replaced orcleaned.
 16. The method of claim 13, further comprising exposing theliquid upstream of the downstream polisher to UV radiation.
 17. Themethod of claim 13, wherein the liquid is ultra pure water.
 18. Themethod of claim 13, further comprises removing TOC upstream from theupstream polisher.
 19. The method of claim 13, wherein the polishers usean ion-exchanging resin with a greater affinity for carbon dioxide thansilicon dioxide.